Add `refactoring` changelog label. Deprecate `improvement`

[carbolymer]

Changelog

- description: |
    Add `refactoring` changelog label. Deprecate `improvement` label
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - improvement    # QoL changes e.g. refactoring
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

We need a label which would describe changes changing only internal structure of the code without any user-facing modifications. This label should also be skipped from changelog.

Checklist

• [ ] Commit sequence broadly makes sense and commits have useful messages
• [ ] New tests are added if needed and existing tests are updated. See Running tests for more details
• [ ] Self-reviewed the diff